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Articles: In Search of Planets

By Girvan McKay

One of the most exciting developments in astronomy within very recent years is the discovery of planets around stars outside our solar system. These are known as extrasolar planets, a term which is inexact but which we shall use for convenience in this article. Until a very few years ago there was no way of knowing whether any such planets existed, and some even wondered whether perhaps our solar system was the only one with planets - or at least, planets on which life was possible. However, when we remember that our Galaxy alone contains more than a hundred thousand million stars, it seems inconceivable that life could have originated nowhere else but on Earth.

In our solar system Earth is, in fact, the only planet where life of any kind is in evidence. Exploratory missions have been sent to all of our neighbouring planets except for Pluto, and so far, no sign of life has been detected elsewhere, although only on Mars has there been a scientific search on the planet's surface.

It appears from our experience that life can arise only where there is liquid water and an atmosphere which has been stable for thousands of millions of years, and that such an environment can exist only on a planet orbiting a sun similar to ours, at a distance not too near and not too far away. We do not yet know whether such planets are a frequent or infrequent phenomenon. However, as from the beginning of 2002, approximately eighty stars are known to have planets. The planets concerned are all gas giants similar to Jupiter, and, of course, quite unsuitable for the emergence of life. Surprisingly, these gas giants are very much closer to their star than Jupiter is to the Sun - in fact almost all of them are closer to their sun that Mercury is to ours.

The main difficulty in locating extrasolar planets lies in the fact that a planet has no light of its own and such reflected light as it receives from its star is completely swamped by the brilliance of the star itself. The attempt to detect an extrasolar planet orbiting even a nearby star would be approximately equivalent to trying to see a coin held up beside a lighthouse at a distance of a thousand kilometres. For this reason it is not possible to observe any such planet using any existing telescope.

The task is not impossible, however. Astronomers have devised seven methods with a view to finding extrasolar planets and possible life. The most fantastic is to search the radio transmissions of civilisations which may have developed on planets with a suitable environment. Members of TAS are probably already familiar with the SETI ("Search for Extra-Terrestrial Intelligence") launched in the United States and still being continued, so far without any tangible success.

The second method is direct observation. With present equipment this is impossible, since a typical star is a million times more brilliant than any nearby planet.

The third method applies only in very special cases, notably for planets orbiting pulsars - rare objects that emit very regular radio pulses. An interruption in the regularity of the pulses may suggest the presence of a planet.

A fourth method is based on an effect known as gravitational lensing, i.e. the curving of light rays by the gravity exerted on a heavy object, whether star or planet. If such an object passes between us and a very distant light source, gravitational lensing can amplify the brilliance of the source for a short time when it is in exactly the right position. Such amplification has not yet been detected in the case of any planet-like object.

A fifth method is known as transit. Transit means that a planet passes exactly between us and the sun and obscures a small part of the sun's light. In our solar system this occurs in the case of Venus and Mars. It is much less easily observable in the case of other stars, but it has had a limited success, e.g. in measuring the presence of chemical elements in the atmosphere of extrasolar planets.

The sixth method uses the changes that a planet causes in the position of the star which it orbits. Every star moves in a more or less circular orbit around the centre of the Galaxy, but it also has its own system of movement in relation to our Sun. Because of the gravitational influence of an orbiting planet, this movement is not in a straight line but in a wavy line which corresponds to the period of the planetary orbit round the star.

The seventh method is similar to the sixth but uses the changes in the speed of movement of the star as a result of the gravity of a nearby orbiting planet. Up to now this method has proved to be the most successful.

In the past decade astronomers have detected several extrasolar planets taking advantage of this method and of the Doppler effect, as follows:

The star and the planet describe a circular orbit around their common centre of mass. The orbits are observed obliquely from a position on Earth. There is seen to be an alternate change in the observed wavelength: towards the red (lengthening) when the star is receding, and towards the blue when the star is coming nearer to us in its orbit. The bigger a planet and the nearer it is to our Sun, the greater its gravitational pull and the periodic change in the speed of the star. Unseen planets have thereby been detected. It is expected that as technology becomes more and more developed it will become ever easier to detect extrasolar planets and eventually to discover whether any such planets can and do sustain life. From there it will be still greater jump to find out whether any such life is intelligent or whether it is nothing higher than lichens and bacteria. However, many of us believe that some form of extraterrestrial life will eventually be found, though maybe not Little Green Men or Star Trek characters with latex masks and a Hollywood accent.

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